BACTERIA 233 



disease. Several diseases are now known that are communi- 

 cated to man and other animals solely through the bites of 

 insects. In the body, bacteria may break down the cells, or 

 they may produce substances, known as toxins which rapidly 

 poison it. In some cases, a colony of bacteria in a limited 

 part of the body may produce toxins so deadly as to cause 

 death in a short time. 



199. Antitoxins. In most cases, when a body is being 

 poisoned by toxins, the cells form antitoxins to counteract 

 them. The antitoxins kill the bacteria and so free the body 

 from their effects. The white corpuscles of the blood also 

 rapidly destroy bacteria. Were it not that the body is pro- 

 tected in this way, even slight injuries would prove fatal. 

 In some cases, the body does not produce its antitoxin fast 

 enough and is helped in its work by the injection of similar 

 antitoxins taken from other animals. The antitoxin used in 

 diphtheria is of this nature. There are a number of bacterial 

 diseases which we usually have only once, the effects of the 

 antitoxins made during the course of the disease seeming 

 to protect the body from other attacks of it through life. 

 The individual is then said to be immune to that disease. 

 In other diseases, the effects fail after a time and the disease 

 may then be taken again. A number of serums which are 

 practically antitoxins have been discovered in recent years 

 and used for the cure of disease. These are made from the 

 blood of other animals attacked by the disease or, in some 

 cases, from the bacteria themselves. 



200. Disinfectants and Antiseptics. Since bacteria are 

 plants, they may be destroyed by the same agencies that kill 

 other plants. Drying stops the activities of all but does not 

 kill many kinds. They simply go into a resting condition and 

 revive when more moisture is found. Exposure to sunlight 

 kills many species and exposure to extreme cold, like drying, 

 retards their activities and in many cases kills them. The 



